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Like so much of Magento — the names of these new nodes hide their purpose from developers. All the <argument/> nodes do ensure a value will be set on the final block object. i.e. The above code will set a label property on the tracking-info-link block (instantiated from the PHP class Magento\Shipping\Block\Tracking\Link). This means you can access the property from your phtml template like this

$block->getLabel();
$block->getData('label');

You can see this in the Magento_Shipping::tracking/link.phtml template file.

Behind the scenes, this functionality is implemented by Magento looking for these argument nodes, and then passing additional information into the Block class when instantiating a Block object. That’s why these nodes are called <arguments/> and <argument/> — because the system designer was focused on the implementation details of the system instead of how end user programmers would use the system.